In the release paper art, there are known a variety of silicone release compositions for use in preventing any sticking or adhesion between substrates of paper, plastics or the like and pressure-sensitive adhesives (PSAs). Of these, solvent type silicone compositions have been widely used because of release properties and relatively wide substrate selectivity.
However, from the standpoints of environmental pollution, safety and health, it now becomes necessary to take countermeasures to reduce the amount of solvent used or to recover the solvent to stop discharge to the exterior. One effective means of reducing the amount of solvent used is use of solventless silicone compositions. In order to apply the solventless silicone compositions to paper, laminate paper or plastic film substrates uniformly to a submicron thickness, an expensive applicator and a sophisticated technique are necessary. In general, most manufacturers hesitate to accept the change from the solvent type silicone to the solventless type silicone.
Another effective means of reducing the amount of solvent used is use of emulsion type silicone compositions. Silicone compositions of emulsion type are known and used in the art. For example, Patent Document 1 discloses a composition obtained by mixing an emulsion composed of an organovinylpolysiloxane, a platinum compound, an emulsifying agent and water with another emulsion composed of an organohydrogenpolysiloxane, an emulsifying agent and water. Also known are a silicone composition produced by emulsion polymerization (see Patent Document 2), and a composition obtained by emulsifying an organovinylsiloxane and an organohydrogenpolysiloxane with a specific emulsifying agent and mixing the emulsion with an emulsion of a platinum compound (see Patent Document 3).
Since these emulsion type silicone compositions can be diluted with water to any desired concentration, an expensive applicator and a sophisticated technique for thin film coating are not necessary as opposed to the solventless type, and their ease of application is approximate to the solvent type.
However, emulsion type silicone compositions have not enjoyed widespread use because of drawbacks associated with water as the dispersing medium. One drawback is that high temperature cure is necessary due to the latent heat of water for evaporation, indicating poor curability as compared with the solvent and solventless types. Another drawback is that wettability and adhesion to substrates are poor because of the surface tension of water. These drawbacks become serious especially with plastic film substrates. They account for the reason why few emulsion compositions have been utilized.
A number of proposals have been made to solve these problems. For example, Patent Document 4 uses an organopolysiloxane having alkenyl radicals at molecular ends, and Patent Document 5 discloses blending of an emulsion of a non-silicone polymer. Most of these compositions are intended to apply to paper substrates, and fail to achieve satisfactory adhesion when applied to plastic film substrates. Under the circumstances, the inventors proposed in Patent Document 6 a silicone emulsion composition based on an organopolysiloxane wherein trifunctional siloxane units (T units) account for 35 to 60 mol % of the entire siloxane units and alkenyl groups account for at least 20 mol % of the entire organic groups, the composition being fully adherent to various plastic film substrates. Although this composition is fully adherent to various plastic film substrates, it must contain at least 50% by weight of the organopolysiloxane containing much trifunctional siloxane units and alkenyl groups, as adhesion-providing component. It is thus difficult to design this composition so as to be lightly or easily peelable from acrylic PSAs.
Recently, release films based on plastic film substrates are often used in the optical and electric/electronic part applications. There is an increasing tendency for these release films to require light removal from PSA. None of currently available silicone emulsion compositions for plastic film substrates meet the current demand.
In general, addition cure silicone emulsion compositions containing organohydrogenpolysiloxane have one common drawback that since the organohydrogenpolysiloxane is in contact with water and surfactant in the emulsion, it often undergoes dehydrogenation during shelf storage or in treating bath. Once dehydrogenation reaction occurs, the amount of active organohydrogenpolysiloxane in the composition is reduced, causing a reduction of cure and adhesion. Particularly when plastic film is used as the substrate, the degradation of organohydrogenpolysiloxane as crosslinker by dehydrogenation largely affects the adhesion of cured silicone coating to the substrate. Therefore, when the addition cure silicone composition is formulated as emulsion, it is desired that the addition cure silicone emulsion composition undergo little dehydrogenation during aging or in treating bath.